In recent years, an energy saving trend on industrial products has been strong in order to deal with the environmental problem, the problem of rise in the price of crude oil, etc. In response to the trend, hybridization using an engine and an electric motor together has been also examined in construction machines whose mainstream is a system in which a hydraulic drive system is heretofore driven only by an engine. By hybridization of a construction machine, it is possible to expect not only the effect of reduction in exhaust gas but also the effect of energy saving, including engine's high efficiency drive, improvement of efficiency in transmission, and recovery of regenerative electric power.
In the background art, as a hybrid construction machine of this type, there has been proposed a hybrid construction machine, in which an electric motor (generator motor) is driven to engage in motoring or regenerating in accordance with power absorbed by a hydraulic pump so as to change the power distribution between an engine and the electric motor and operate the engine in a high-efficiency range in order to reduce the fuel consumption (for example, see Patent Literature 1).